1. Field of the Invention
The present patent application relates to an image processing device adapted to devices such as a facsimile machine, a printer, a scanner, a copier, a multifunctional machine, and a personal computer; an image forming apparatus such as a facsimile machine, a printer, a scanner, a copier, and a multifunctional machine; an image processing method performed by the image processing device and the image forming apparatus; and a program executed by the image processing device and the image forming apparatus.
2. Discussion of the Background Art
As a document reading method performed by an image reading device, a method has been commonly used which illuminates a document with the light emitted from, for example, a xenon lamp serving as a light source, and causes an image sensor to read the reflected light from the document through a reduction optical system.
Recently, however, a so-called contact image sensor (CIS) has started to be widely used for the purpose of reducing the size of a device. In a CIS, which uses a small-sized LED as a light source, an image is directly read by a linear sensor through a SELFOC™ lens, for example (SELFOC is a registered trademark of NSG America, Inc.). Further, a so-called multi-chip image sensor array has come to be widely used as the image sensor used in the CIS as described above, in which photoelectric conversion sensor chips are arranged in multiple lines in the main scanning direction. One problem with the multi-chip, image sensor array, however, is that the chips forming the array have different characteristics. As a result, in the read image of a document, therefore, color irregularities occur at intervals corresponding to the widths of the chips. In extreme cases, a striped pattern may appear in the read image data.
A background technique proposes to read a test chart by using an image sensor array, calculate a color correction coefficient for each of chips, and then perform conversion for each of the chips by setting one color space as the target.
Further, in recent years, an image reading device has started to be widely used which uses an automatic document feeder (ADF) to read both surfaces of a document substantially simultaneously (i.e., not exactly simultaneously, but during the same document conveying operation). For example, in a simultaneous duplex reading operation by the image reading device according to another background technique, when the image reading device reads a document using a fixed optical system during conveyance of the document, the image reading device reads one side of the document by using a reading device called CIS which includes a multi-chip image sensor, and reads the other side of the document by using a reduction optical system which includes a charge-coupled device (CCD) as an image sensor.
The above-described first background technique, however, exhibits insufficient color correction among chips, and thus causes color irregularities. Even if the accuracy of the color correction among chips is improved by a calibration technique and so forth, it is difficult to perfectly adjust the respective colors of the chips to an intended color, and thus an error occurs. In particular, if the output color fluctuates in a low-chroma area, and if the color is reproduced with a chroma higher than an intended chroma, humans in general easily recognize such fluctuation as color irregularities. The nature of human vision is such that humans find a color change more easily in an achromatic color than in a chromatic color, and thus are more sensitive to a change occurring in an achromatic area.
Meanwhile, in the above-described second background technique, if processing to suppress the color irregularities attributed to the multi-chip image sensor is performed on the CCD image sensor, the chroma of a low-chroma area is unnecessarily reduced. Therefore, it is generally considered that chroma conversion processing to suppress color irregularities should be avoided in the reading operation performed by the CCD image sensor.
However, the chroma in a low-chroma area is different between the respective images of the front and rear surfaces of a document read by the simultaneous duplex reading operation using the ADF, and between the image read by the simultaneous duplex reading operation and the image read by a book-reading operation of reading a fixed document, i.e., a method of reading a document fixed on a contact glass by scanning the document with the use of an optical reading system. Such a difference in chroma may be recognized as a difference in image. Therefore, some users may rather want to perform equal chroma conversion processing in both the reading operation by the multi-chip image sensor and the reading operation by the CCD image sensor.